Missing y label

I did read that fine file.
I added the following line to my /etc/apt/sources.list file:

tmt-local2023% tail -2 /etc/apt/sources.list
# R sources
deb https://mirror.las.iastate.edu/bin/linux/ubuntu xenial/

Here is what apt says on my machine

tmt-local2024% apt-cache policy r-base-core r-base-core: Installed: 3.2.3-4 Candidate: 
3.2.3-4 Version table: *** 3.2.3-4 500 500 http://us.archive.ubuntu.com/ubuntu 
xenial/universe amd64 Packages 100 /var/lib/dpkg/status My long term habit is to stick to 
LTS versions, which currently is 16.04: "still running 16.10" is a different outlook. The 
system admins here tend to be more conservative than me. Again, thanks for any input. 
Getting this particular box straightened out quickly isn't critical, but I would like to 
understand the root of the issue. Terry T.

Read the fine README here:https://cloud.r-project.org/bin/linux/ubuntu/README.html
Similar one for Debian athttps://cloud.r-project.org/bin/linux/debian/

Many of us have been using R from CRAN on Ubuntu and Debian for many years,
and many update cycles.  I try hard to get new R versions into Debian the
morning of the release [1] and Michael and Johannes usually produce binary
for the mirrors within a day or two too. On this box still running 16.10:

edd at bud:~$ apt-cache policy r-base-core
r-base-core:
   Installed: 3.4.1-2yakkety0
   Candidate: 3.4.1-2yakkety0
   Version table:
  *** 3.4.1-2yakkety0 500